AbstractThe molecular weight distribution (MWD) of high‐molecular‐weight isotactic polypropylene is measured by a new method developed for polyethylene (PE) using the turbidity at the lower critical solution temperature (LCST). The attenuation of transmitted light hi is measured at Ti as each fraction i is separated from the solution during a temperature increase ΔTi. From the thermogram, the set of hi (Ti), the MWD can be calculated through a relation Mi = f(Ti). The solvent chosen, a mixture of butane and 2‐methylbutane, optimizes resolution and relatively rapid dissolution. Homogenization and absence of degradation are verified by different methods including calorimetry in solution. © 1992 John Wiley & Sons, Inc.